I rebuilt the motor at 230k miles because my wife overheated it when the radiator got a leak. It now has 250k miles and started missing when I start it until the motor reaches operating temp. the only code shows misfire cylinder #1. Changed plugs, also used a friends coil pack bar to test mine and no change. Does this motor have a second coolant temp sensor that helps decide the air fuel mixture until the motor warms up? I've had a different make of engine have that on it before... As soon as engine warms up, miss goes away. Any thoughts?

 

The only thing I've noticed on my 2000 4.0 is if the engine is warm but the exhaust is cold, like restarting after 10 or 15 minutes, it runs rough until (I sort of figured out) the 02 sensor rewarms.

When you rebuilt the motor, did you replace the injectors? It's possible your #1 is leaking or sticking until it's warm. If you want to go to the trouble, you can swap it with another and see if your misfire moves. You can probably get another set from the boneyard. Bustedback used some from a Dodge Neon and was very happy with the results.

Welcome to CF! There is a lot of good info here and friendly people. Just remember that it is an opinion site and not all responses are factual.

 

Good call on the injector. I moved #1 to #4 and the misfire moved to cylinder 4. I put Lucas injector cleaner in just for kicks and put new orings and cleaned the tips on all 6. Has anyone taken these injectors apart? Or is it easier to just buy new ones? I always has the assumption that injectors were either bad or they were good. I wouldn't have thought it would have quit missing when the motor warmed up. Thanks!!!

 

No, the injectors are pretty much a sealed unit. A lot of guys just get some from the junkyard and have good luck with them. As I said, one of our senior members, Bustedback, got a bunch from a Dodge Neon and they proved to give him better power and performance. Others used injectors from a Ford something or other with the same result. Busted mentioned something about cargo pants, whatever he meant by that you can draw your own conclusions.

You can buy reconditioned ones or new also. I've heard that even reconditioned ones are a little expensive. They CAN be cleaned, but you need to pump a good quality solvent thru them at a high pressure. I think yours got cooked when it overheated.